Large quantities of methane, the main component of natural gas, are available in many areas of the world. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make the use of this remote gas economically unattractive. To improve the economics of natural gas use, much research has focused on the use of methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids.
As a result, various technologies for the conversion of methane to hydrocarbons have evolved. The conversion is typically carried out in two steps. In the first step methane is reformed with water or partially oxidized with oxygen to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas is converted to hydrocarbons.
This second step, the preparation of hydrocarbons from synthesis gas is well known in the art and is usually referred to as Fischer-Tropsch synthesis, the Fischer-Tropsch process, or Fischer-Tropsch reaction(s). The Fischer-Tropsch reaction involves the catalytic hydrogenation of carbon monoxide to produce a variety of products ranging from methane to heavy hydrocarbons (up to C80 and higher) as well as a variety of oxygenated hydrocarbons. The methanation reaction was first described in the early 1900's, and the later work by Fischer and Tropsch dealing with higher hydrocarbon synthesis was described in the 1920's.
Catalysts for use in such synthesis usually contain a catalytically active metal of Groups 8, 9, 10 (in the New notation of the periodic table of the elements, which is followed throughout). In particular, iron, cobalt, nickel, and ruthenium have been used as the catalytically active metals. Cobalt, iron and ruthenium have been found to be most suitable for catalyzing a process in which synthesis gas is converted to primarily hydrocarbons having five or more carbon atoms (i.e., where the C5+ selectivity of the catalyst is high). Additionally, the catalysts often contain one or more promoters and a support or carrier material. Ruthenium is a widely used promoter for cobalt catalysts.
The Fischer-Tropsch synthesis reactions are highly exothermic and reaction vessels must be designed for adequate heat exchange capacity. Because the feed streams to Fischer-Tropsch reaction vessels are gases, while the product streams include liquids, the reaction vessels must have the ability to continuously produce and remove the desired range of liquid hydrocarbon products. The first major commercial use of the Fischer-Tropsch process was in Germany during the 1930's. More than 10,000 B/D (barrels per day) of products were manufactured with a cobalt based catalyst in a fixed-bed reactor. This work was described by Fischer and Pichler in German Patent 731,295 issued Aug. 2, 1936.
Motivated by the hope of producing high-grade gasoline from natural gas, research on the possible use of the fluidized bed for Fischer-Tropsch synthesis was conducted in the United States in the mid-1940s. Based on laboratory results, Hydrocarbon Research, Inc. constructed a dense-phase fluidized bed reactor, the Hydrocol unit, at Carthage, Tex., using powdered iron as the catalyst. Due to disappointing levels of conversion, scale-up problems, and rising natural gas prices, operations at this plant were suspended in 1957. Research continued, however, on developing Fischer-Tropsch reactors, such as slurry-bubble columns, as disclosed in U.S. Pat. No. 5,348,982. Despite significant advances, certain areas of the Fischer-Tropsch technology still have room for improvement. One potential technology in need of improvement relates to regeneration of the Fischer-Tropsch catalyst.
After a period of time in operation, a catalyst will become deactivated, losing its effectiveness for synthesis gas conversion to a degree that makes it uneconomical at best and inoperative at worst. At this point, the catalyst can be either replaced or regenerated. Because the catalysts tend to be relatively expensive, regeneration is preferred over replacement. Catalyst systems can become deactivated by a number of processes, including coking, sintering, oxidation, and poisoning. The process chiefly responsible for deactivation varies among catalyst systems. Therefore, the preferred method for regeneration tends to depend on the catalyst system to be regenerated.
Research is continuing on the development of more efficient Fischer-Tropsch catalyst systems and catalyst systems that can be more effectively regenerated. In particular, a number of studies describe the use of various gases, including hydrogen, air, and carbon monoxide to regenerate a variety transition metal containing Fischer-Tropsch catalyst systems.
U.S. Pat. No. 3,958,957, issued on May 25, 1976, describes a carbon-alkali metal catalyst, used for conversion of synthesis gas to methane and higher hydrocarbons at a pressure of 100-1500 psig and a temperature of 300-550° F. at a typical gas hourly space velocity of 1000 volumes gas/hr/volume catalyst. The carbon-alkali metal catalyst can be regenerated with air oxidation.
U.S. Pat. No. 4,151,190, issued on Apr. 24, 1979, describes a catalyst comprising at least one of a sulfide, oxide, or metal of Mo, W, Re, Ru, Ni, or Pt, at least one of a hydroxide, oxide, or salt of Li, Na K, Rb, Cs, Mg, Ca, Sr, Ba, or Th, and a support, used for conversion of synthesis gas with an H2:CO ratio of 0.25-4.0, preferably 0.5-1.5, to C2-C4 hydrocarbons at a pressure of 15-2000 psia and a temperature of 250-500° C. at a typical gas hourly space velocity of 300 v/hr/v. This catalyst can be regenerated by contacting it with hydrogen gas at 500-600° C. for 16 hours.
U.S. Pat. No. 4,738,948, issued on Apr. 19, 1988, describes a catalyst comprising cobalt and ruthenium at an atomic ratio of 10-400, on a refractory carrier, such as titania or silica. The catalyst is used for conversion of synthesis gas with an H2:CO ratio of 0.5-10, preferably 0.5-4, to C5-C40 hydrocarbons at a pressure of 80-600 psig and at a temperature of 160-300° C., at a gas hourly space velocity of 100-5000 v/hr/v. This catalyst can be regenerated by contacting it with hydrogen gas at 150-300° C., preferably 190-260° C., for 8-10 hours.
U.S. Pat. No. 5,728,918, issued on Mar. 17, 1998, describes a catalyst comprising cobalt on a support, used for conversion of synthesis gas with an H2:CO ratio of 1-3, preferably 1.8-2.2, to C5+ hydrocarbons at a pressure of 1-100 bar and at a temperature of 150-300° C., at a typical gas hourly space velocity of 1000-6000 v/hr/v. This catalyst can be regenerated by contacting it with a gas containing carbon monoxide and less than 30% hydrogen, at a temperature more than 10° C. above Fischer-Tropsch conditions and in the range 100-500° C., and at a pressure of 0.5-10 bar, for at least 10 minutes, preferably 1-12 hours. The contact time period depends on temperature and gas hourly space velocity. The U.S. Pat. No. 5,728,918 also teaches an activation procedure, which may include a first step of contacting the catalyst with a gas containing molecular oxygen, preferably air, at 200-600° C., at atmospheric pressure, for more than 30 minutes, and preferably for 1-48 hours.
U.S. Pat. No. 4,595,703, issued on Jun. 17, 1986, describes a catalyst comprising cobalt or thoria promoted cobalt on a titania support, used for conversion of synthesis gas with an H2:CO ratio of 0.5-4, preferably 2-3, to C10+ hydrocarbons at a pressure of preferably 80-600 psig, and at a temperature of 160-290° C., at a gas hourly space velocity of 100-5000 v/hr/v. This catalyst can be regenerated by contacting it with hydrogen gas, or a gas which is inert or non-reactive at stripping conditions such as nitrogen, carbon monoxide, or methane, at a temperature substantially the same as Fischer-Tropsch conditions. If it is necessary to remove coke deposits from the catalyst, the catalyst can be contacted with a dilute oxygen-containing gas, at oxygen partial pressure of at least 0.1 psig, at 300-550° C., for a time sufficient to remove coke deposits, followed by contact with a reducing gas containing hydrogen, at a temperature of 200-575° C. and at a pressure of 1-40 atmospheres, for 0.5-24 hours.
U.S. Pat. No. 4,585,798 issued on Apr. 29, 1986, describes a catalyst comprising cobalt and ruthenium in an atomic ratio greater than about 200:1 and, preferably, a promoter, such as a Group IIIB or IVB metal oxide, on an alumina support, used for conversion of synthesis gas to hydrocarbons at a pressure of preferably 1-100 atmospheres and at a temperature of 160-350° C., at a gas hourly space velocity less than 20,000 v/hr/v, preferably 100-5000 v/hr/v, especially 1000-2500 v/hr/v, which is activated prior to use by reduction with hydrogen gas, followed by oxidation with diluted air, followed by further reduction with hydrogen gas.
Despite the vast amount of research effort in this field, currently known methods of regeneration of Fischer-Tropsch catalysts are not always sufficiently effective for a particular catalyst system. Among the main deactivation mechanisms for cobalt based catalysts are sulfur poisoning [e.g. R. L. Espinoza, et al, Applied Catalysis A:General 186 (1999)13], metal oxidation [e.g. D. Schanke et al, Catal. Lett. 34 (1995) 269] and surface condensation of heavy hydrocarbons [e.g. E. Iglesia et el, J. Catal. 143 (1993) 345]. The removal of heavy hydrocarbons, deposited in the pores of a used catalyst, is therefore one of the challenges to efficient commercialization of slurry bed technology for the Fischer-Tropsch reaction.
In a slurry bed reactor, the Fischer-Tropsch catalyst particles are suspended in liquid reaction products (heavy hydrocarbons), predominantly wax. These heavy hydrocarbons may include heavy hydrocarbons formed in the Fischer-Tropsch reaction. In a fixed bed reactor, the catalyst particles, though not suspended in heavy hydrocarbons, will contain and/or become coated with heavy hydrocarbons as reaction proceeds. One of the deactivation mechanisms of the catalyst is the deposition of very heavy hydrocarbons into the catalyst pores and/or on the surface of the catalyst particles. Hydrogen gas, conventionally maintained at reaction pressure, has been used to remove a portion of this material through hydrogenation of the heavy hydrocarbon. However, this method has the disadvantages that hydrogenation may be incomplete and that the hydrogenated hydrocarbon may remain deposited in the pores of the catalyst and/or on the surface of the catalyst particles. Also, a certain degree of hydrogenolysis, that is, destruction of valuable heavy hydrocarbons may occur, producing gaseous hydrocarbons of lower commercial value.
Hence, there is still a great need to identify new regeneration methods which can be used concurrently and/or periodically with contacting regenerated catalyst with synthesis gas, so as to maximize the regenerated catalyst activity and thus enhance the process economics.